After retrieving the Elder Scroll (Blood) from the Soul Cairn, check the progress of the quest "Seeking Disclosure" in your mission log:

Seeking DisclosureIn order to locate Auriel's Bow, the Moth Priest, Dexion, indicated that two other Elder Scrolls would be required. The first scroll foretells the interactions of the dragons with the history of Nirn. The second foreshadows the defiance of the gods with the blood of the mortals.

If you already did the main quest, but sold the Elder Scroll to Urag gro-Shub, you will be given another quest that will give you a chance to get the Elder Scoll back.

Scroll ScoutingIn order to discover the location of Auriel's Bow, I need to acquire the Elder Scroll that foretells the interactions of the dragons with the history of Nirn. In order to begin my search, I've been told to ask at the College of Winterhold.

(or)

Having sold that particular scroll to Urag gro-Shub at the College of Winterhold, I'll need to return there to see if he's willing to sell it back.

ObjectivesDiscover the location of the Elder Scroll (Dragon) / Buy back the Elder Scroll (Dragon) from Urag gro-ShubGo to the College of Winterhold and talk to Urag Groshub (Usually in the Arcaneum). There will be an additional dialog options:"I want to buy back the Elder Scroll I sold you"

If you are buying the scroll back, he wants 4,000 gold for it. However, if you are the Arch Mage, you can ask him to sell it to you for less. You should see the following option:"I'm the Arch-Mage. It's 2,000 coins"

Once you get the Elder Scroll (Dragon) it will complete the quest "Scroll Scouting".

The quest "Seeking Disclosure" will also progress if finding the Dragon Elder Scroll after already finding the Blood Elder Scroll:

Seeking DisclosureI've secured both of the Elder Scrolls needed to fully decipher the Tyranny of the Sun and pinpoint Auriel's Bow. I need to return to Castle Volkihar where Dexion is imprisoned so he can begin his reading.

Your objective now will be to bring both Elder Scrolls to Dexion. Fast Travel to Castle Volhikar, and talk to Dexion inside. Tell him that you have brought him the Elder Scrolls.

This completes the quest "Seeking Disclosure", and starts the next quest, "Unseen Visions".